The Role of Dynamic Stabilization and the Aging Spine

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56 The Role of Dynamic Stabilization and the Aging Spine

Reginald J. Davis

KEY POINTS

• The rationale behind posterior motion preservation devices is explained.
• Devices for specific clinical indications are shown.

Introduction

Posterior dynamic stabilization of the lumbar spine is an evolving field. At the core is the concept of incremental stabilization or progressive control of motion of the spine. This contrasts with more traditional fusion techniques where stabilization entails rigid fixation with abolition of motion. These goals are achieved with surgical implantation of posteriorly placed devices, often following surgical decompression where indicated. These devices exert varying degrees of motion control at varying anatomical regions of influence, based on design.

It is a rapidly expanding field with a broad spectrum of devices, approaches, materials, and degree of imparted stabilization. Proper discussion of dynamic stabilization can only occur with organizational hierarchy, considering anatomical location, functional impact, intensity of intervention, and targeted goal of intervention. Each will be considered with relationship to the others.

Devices

The devices can be largely grouped into three categories, based on anatomical location of implantation. These categories are interspinous, facet, and pedicle. These discrete anatomical insertion points allow for a targeted intervention with very specific actions and indications. There is sufficient overlap such that a broad spectrum of disease can be treated.

Interspinous Spacers

The spinous processes and the interspinous space are the sites used with increasing frequency in treating spinal conditions. Though few conditions directly affect the spinous process itself, this anatomical location has utility in implant attachment for distraction and stabilization to a significant degree.

The rationales for interspinous devices are several. They distract the neural foramen to reduce nerve root compression. They also share load with the posterior disc, unload the facets, and assist with stability at the operated level. For the most part they are minimally invasive and easily revisable.

These devices are surgically implanted between the spinous processes or within the interlaminar space. In this location they act to control extension, unload the facets, tension the posterior annulus, as well as tension the posterior ligaments. This influence can be enhanced with surgical positioning in more flexion, or after loaded distraction. The resulting kyphosing moment can increase overall volume for neural elements.

X-Stop (Kyphon)

X-Stop is a titanium alloy device that is designed to stop extension (Figure 56-1). The oval spacer conforms to the interspinous space, and the wings prevent lateral migration. It is minimally invasive and inserted laterally, thus preserving the supraspinous ligament. It is designed to be implanted under local anesthesia. In clinical trial, X-Stop was significantly better than nonoperative treatment of lumbar spinal stenosis at 1 and 2 years post-op. The observed success rate was comparable to published reports for decompressive laminectomy, but with considerably lower morbidity.1,5 X-Stop is currently approved by the U.S. Food and Drug Administration (FDA) for use in patients with claudication symptoms from lumbar stenosis. Patients with relief in flexion and who are otherwise comfortable sitting respond well.

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FIGURE 56-1 X-Stop device interspinous spacer (Kyphon).

Wallis (Zimmer Spine)

The Wallis device is a PEEK interspinous spacer secured to the spinous processes using PET bands (Figure 56-2). It is designed to block extension and control flexion. Indications include isolated lumbar intervertebral instability, such as herniated discs, Modic I degenerative lesions, degenerative disc disease at a level adjacent to a previous fusion, and spinal stenosis treated without laminectomy. In long term (13 year) OUS follow-up, the device obviated the need for arthrodesis in 80% of patients.4 The Wallis device is currently undergoing clinical evaluation and is not FDA approved.

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FIGURE 56-2 The Wallis device interspinous spacer (Zimmer Spine).

Diam (Medtronic)

Diam is an interspinous stabilizer composed of a silicone bumper, encased in polyester mesh, and secured with polyester sutures (Figure 56-3). It is inserted with minimal access and minimal tissue disruption. It is designed to resist extension and reduce intradiscal pressure. Indications include restoration of early segmental degeneration, correction of misalignment often seen in discectomy, and stenosis. Diam is currently undergoing clinical evaluation and is not FDA approved.

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FIGURE 56-3 The Diam interspinous stabilizer (Medtronic).

Coflex (Paradigm)

Coflex is a titanium alloy device whose U-shaped body conforms to the interspinous and interlaminar space (Figure 56-4). It acts as a stiff spring, dynamically stabilizing extension. Lateral migration is prevented by wings compressed onto the spinous processes. Indications include lumbar stenosis, adjacent segment disease, recurrent HNP and early symptomatic disc degeneration. In OUS implantation after decompressive laminectomy in degenerative lumbar spinal stenosis, Coflex was found to be less invasive and provided similar clinical outcome in comparison with instrumented fusion.2 Coflex is currently undergoing clinical evaluation and is not FDA approved.

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FIGURE 56-4 The Coflex device (Paradigm).

ExtenSure (NuVasive)

ExtenSure is a PEEK interspinous spacer secured by geometric conformity to the anatomy and the suturing of the supraspinous ligament (Figure 56-5). It is designed to alleviate pseudoclauditory symptoms and radicular symptoms by enlarging the central canal, lateral recess, and foramina. It also decreases or eliminates low back pain by decreasing pressure on arthritic facet joints, degenerative discs, or both. It also restores disc height and foraminal volume. It is indicated in moderate to severe spinal stenosis, degenerative spondylolisthesis, and mild to moderate degenerative scoliosis. ExtenSure is currently undergoing clinical evaluation and is not FDA approved.

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FIGURE 56-5 The ExtenSure interspinous spacer (NuVasive).

In-Space (Synthes)

In-Space is a laterally placed PEEK cylindrical device, secured by deployable wings (Figure 56-6). It is intended to stop segmental extension and to distract the symptomatic interspinous space. In doing so, maintenance of foraminal height, opening of the area of the spinal canal, reduction of stress on the facets, and relieving of pressure on the posterior annulus results. It is indicated in lumbar spinal stenosis, disc protrusions with discogenic low back pain, facet syndrome due to face osteoarthritis, degenerative spondylolisthesis up to grade 1, and degenerative disc disease. In-Space is not FDA approved.

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FIGURE 56-6 The In-Space device (Synthes).

Superion (Vertiflex)

Superion is a titanium alloy interspinous device with deployable wings (Figure 56-7). It is designed for percutaneous implantation in the treatment of moderate degenerative lumbar stenosis at one or two levels. Superion is currently under clinical investigation in the United States and is not FDA approved.

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FIGURE 56-7 The Superion interspinous device (Vertiflex).

Facet Devices

The facet poses unique challenges. The anatomy and functional interrelationships are complex. The 3-D sliding synovial articulation is difficult to replicate. Physiologically, the facets have nociceptive and proprioceptive input that is vital to the proper function of the motion segment. The resulting chemical and mechanical pain generation is difficult to treat surgically. Kinematically complex, facet joints engage in coupled shear and sliding, as well as rotational load sharing, all of which can be specific to certain levels.

This results in a very complex continuum of disease. Painful inflammation, osteoarthritis, stenosis, abnormal loading, and total failure of the functional spinal unit can all originate with facet disease. These challenges are not easily overcome, addressed, or surgically treated. Facet arthroplasty, a rapidly evolving subspecialty in motion preservation, strives to address these issues in the most ergonomic manner.

The overarching goal is reduction of pain and the return of function. The design rationales for the facet-based devices are to clinically correlate intensity of intervention to severity of disease, to allow or restore more physiological loading, to allow or restore the physiological center of rotation, and to control the range of motion of the motion segment.

The emerging technologies are thoughtfully engineered. They are well studied and modeled as the result of extensive research and analysis. The facet-based devices vary from resurfacing, through augmentation and partial replacement, to total replacement. These devices are designed to complement residual kinematics of the motion segment, or to duplicate native motion in total replacement.

Zyre (Quantum Orthopedics)

Zyre is an interpositional arthroplasty device (Figure 56-8). It consists of a cobalt chromium intraarticular spacer, through which passes a PET cord with chromium retainers. It is minimally invasive, requires no bone resection, maintains capsular integrity, and can be implanted with or without decompression. Indications include painful degeneration of the facet with failed CMM. Advantages include minimal disruption of anatomy and multiple revision options. This concept is early technology with a paucity of clinical data. Zyre is not FDA approved.

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FIGURE 56-8 The Zyre interpositional arthroplasty device (Quantum Orthopedics).

Fenix (Gerraspine AG)

Fenix is a cobalt chromium facet joint resurfacing device. It has superior and inferior components secured with a translaminar locking screw (Figure 56-9). It is designed to eliminate the painful components, resurface the facet joint, preserve supporting structure and anatomy, and allow or restore physiologic motion. Surgery entails removal of capsule and intra-articular cartilage, with or without decompression. Indications include significant facet disease and subarticular stenosis. Fenix has had limited clinical use and is not FDA approved.

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FIGURE 56-9 The Fenix facet joint resurfacing device (Getraspine AG).

Anatomic Facet Replacement System (Facet Solutions)

Anatomic Facet Replacement System (AFRS) is a total facet replacement device composed of cobalt-chromium-molybdenum articular surfaces that uses conventional pedicle screw fixation (Figure 56-10). It is designed to reproduce facet anatomy and preserve or restore natural lumbar biomechanics. Surgical implantation is through a midline approach and entails total facetectomy. Indications include osteoarthritis of the facets causing stenosis, and low grade degenerative spondylolisthesis. Fixation with standard pedicle screws allows ready revision options. AFRS I is in clinical trial in the United States and is not FDA approved.

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FIGURE 56-10 Anatomic Facet Replacement System (Facet Solutions).

Total Facet Arthroplasty System (Archus)

Total Facet Arthroplasty System (TFAS) is a total facet replacement device composed of cobalt-chromium articular surfaces and titanium alloy cross arm assembly (Figure 56-11). It comprises paired cephalad bearings and paired caudal housings in a “ball in cup” motion-constraining configuration. These are supported by the titanium cross arm assembly with cemented pedicle post fixation. In situ modular assembly allows for precise adjustment to individual anatomy. Proposed indications include degenerative disease of the facets, facet instability, up to grade I spondylolisthesis with neurological impairment, central or lateral spinal stenosis, at L3-L4 or L4-L5. TFAS is being clinically evaluated in the United States and is not FDA approved.

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FIGURE 56-11 Total Facet Arthroplasty System (Archus).

Total Posterior System (Impliant)

Total Posterior System (TOPS) is composed of opposing titanium plates with interlocking PCU. It is fixed to the spine with polyaxial pedicle screws (Figure 56-12). The entirety of the posterior elements is totally replaced. Motion is restored and constrained in all planes via polymeric dampening. Utilizing the same surgical technique as standard posterior fusion, it recreates the normal biomechanics of the spine, allowing full physiologic range of motion. Surgery occurs through a midline approach and laterally placed pedicle screws. Total facetectomy and removal of posterior elements is necessary. Implantation requires a precise jig assembly. Indications include moderate to severe spinal stenosis. TOPS is being clinically evaluated in the United States and is not FDA approved.

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FIGURE 56-12 Total Posterior System (Impliant).

Pedicle-Based Dynamic Rods

The pedicle-based devices offer the most secure fixation to the spine and thus the greatest opportunity to control motion. Influence is exerted on facets, posterior ligaments, and posterior disc complex. Implantation can be unloaded utilizing a neutral position of the spine, resulting in passive control of motion, or implantation can be loaded utilizing a more distracted position of the spine, resulting in more dynamic load sharing. These devices may be stand-alone or placed in conjunction with decompression or as an adjunct to fusion. There are many devices, each with unique characteristics. This results in a broad range of control exerted, and thus coverage over a wide spectrum of potential spine pathologies. All share the common goal of alleviating back and leg pain using more flexible constructs and materials to stabilize the spine while preserving anatomical structures.

N-Hance (Synthes)

N-Hance is a flexible posterior stabilizing device that is composed of a collar of paired PCU spacers with interposed titanium ring and end caps (Figure 56-13). This unit slides over the tapered core of a 6-mm titanium rod. The construct provides elongation, compression, and angulation. N-Hance is 510K approved by the FDA as an adjunct to fusion.

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FIGURE 56-13 The N-Hance flexible posterior stabilizing device (Synthes).

Stabilimax NZ (Applied Spine)

Stabilimax NZ is an investigational posterior stabilizing system that utilizes a dual spring configuration to confer physiologic motion parameters (Figure 56-14). It is intended to provide maximum stabilization to the spine, decreasing the abnormal motion that causes pain, while maintaining physiologic motion.

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FIGURE 56-14 The Stabilimax NZ investigational posterior stabilizing system (Applied Spine).

It is designed to provide stabilization of the lumbar spine in patients receiving decompression surgery for the treatment of clinically symptomatic central or lateral spinal stenosis. Stabilimax NZ is currently being clinically evaluated and is not FDA approved.

Dynesys (Zimmer Spine)

The Dynesys Dynamic Stabilization System is a posteriorly placed pedicle-based device (Figure 56-15). It is composed of titanium alloy screws, an interposed PCU spacer, and a through-passed PET cord. During implantation of the device, the cord is placed in tension and the spacer is placed in compression. These opposing vectors of force act through the pedicle screws to dynamically neutralize the spinal segment in flexion, extension, and at rest. The system is 510K approved as an adjunct to fusion. In prospective OUS clinical trial, dynamic neutralization proved to be a safe and effective alternative (to fusion) in the treatment of unstable lumbar conditions.3

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FIGURE 56-15 The Dynesys Dynamic Stabilization System (Zimmer Spine).

Dynesys is currently in FDA panel discussion regarding approval as a nonfusion dynamic stabilization system.

Dynamic TTL-Rod (Scient’x)

The Dynamic TTL-Rod is composed of titanium rods with an interposed damper (Figure 56-16). The damper is a series of washers contained within a bell housing. This configuration results in a posterior rod with micro motion of 2 mm. It is designed to stabilize the spinal segment in semirigid fashion with reduced forces at the bone screw interface. The Dynamic TTL-Rod is 510K approved as an adjunct to fusion.

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FIGURE 56-16 The Dynamic TTL-Rod (Scient’x).

CD Horizon Legacy Peek Rod System (Medtronic)

The CD Horizon Legacy Peek Rod System is a pedicle-based, posterior rod device (Figure 56-17). It is composed of standard polyaxial pedicle screws attached to a peek rod. It is designed to provide semirigid fixation that closely replicates the natural load distribution of the lumbar spine for patients who undergo spinal fusion surgery. The CD Horizon Legacy Peek Rod System is 510K approved as an adjunct to fusion.

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FIGURE 56-17 The CD Horizon Legacy Peek Rod System (Medtronic).

DSS Spine Stabilization System (Paradigm)

The DSS Spine Stabilization System is a pedicle-based posterior coupler device (Figure 56-18). It is an entirely modular system composed of titanium monoaxial pedicle screws, upon which are placed washers and spherical spacers. This allows polyaxial orientation of the couplers. The couplers are made of titanium. The configuration of outer spiral cut housing and inner shaft with a ball-in-socket piston confers motion is all directions with stoppage within physiologic ranges. The hemispherical screw interfaces allow further polyaxial implantation. It is designed to allow physiologic motion within an overall reduced range and to restrict the neutral zone. The DSS Spine Stabilization System is 510K approved as an adjunct to fusion.

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FIGURE 56-18 The DSS Spine Stabilization System (Paradigm).

Dynabolt (VertiFlex)

Dynabolt is a pedicle screw–based flexible posterior rod. It allows for full range of motion and can be delivered percutaneously. It is designed to reduce bone–screw interface stresses and to enable load sharing through the spinal column during motion. Dynabolt is 510K approved as an adjunct to fusion.

Clinical Application

The aging spine demonstrates characteristic degeneration that is unique depending on anatomical location. The ligaments progress from desiccation and loss of elasticity, through inflammation, to end-stage hypertrophy and calcification. The facets suffer inflammation, capsular insufficiency, synovial deterioration, and bony overgrowth. The metabolic and structural deterioration of bone ultimately results in osteopenia, osteoporosis, and mechanical insufficiency. These degenerative processes, individually and in combination, result in a broad spectrum of spinal conditions, which vary in severity, clinical manifestation, anatomical location, and intensity of appropriate intervention. For each of these conditions a dynamic solution can be considered. As this is still a new and evolving field, most proposed interventions are intuitive with a paucity of supporting clinical data.

Ligament

Mild ligamentous disease, beyond simple inflammation, may result initially in ligamentous laxity. This state contributes to mild instability. In addition, thickening or buckling of specific ligaments lining the spinal canal can contribute to spinal stenosis. The interspinous spacers overall may have clinical efficacy in such cases. X-Stop has demonstrated efficacy in these patients. The dynamic rods may also be used but represent a more aggressive solution. The relative invasiveness of pedicle fixation in such cases is overkill in many instances. The facet replacement devices likewise entail more disruption of anatomy than can be recommended for otherwise mild disease.

Moderate ligamentous laxity can result in more significant instability. The abnormal motion, characterized by an increased neutral zone, can result in back pain. A greater degree of central canal stenosis can also be seen. Surgical treatment often entails direct decompression with concomitant worsening of instability. The interspinous devices could be considered in these instances, but for the most part might lack sufficient influence of translation to be effective. A more robust interlaminar device such as Coflex may be used in selected cases. The dynamic rods have much more utility in this patient population. Pedicle-based devices that control the neutral zone include DSS and Stabilimax. Following more extensive laminectomy and partial facetectomy or with greater instability, more rigid devices may be needed. Dynesys, CD Horizon Peek, and Dynamic TTL have sufficient rigidity as to be useful in up to grade 1 spondylolisthesis.

More severe ligamentous dysfunction can result in degenerative spondylolisthesis, more severe central canal stenosis, lateral or subarticular stenosis, and secondary disc failure. The stiffer pedicle-based devices could be considered here. Devices to be considered include Dynesys, Isobar, and PEEK rods. Efficacy is exceeded with greater than grade 1 spondylolisthesis, pars defect, and greater than 50% facetectomy. Fusion with traditional rigid fixation technology is then warranted.

Facet

Mild capsular failure and intractable synovial inflammation may result in facet pain. Facet resurfacing technology, such as Zyre may be efficacious.

Moderate capsular failure and synovial dysfunction also occurs and results in significant pain and disability. Synovial cysts with lateral compression, facet overgrowth with subarticular stenosis, and degenerative subluxation of facets could be treated with facet reconstruction. Fenix and similar technologies may be indicated in such cases.

Severe facet disease results in more severe stenosis, and facet failure. This probably requires facet replacement. Applicable technologies include TFAS, Facet Solutions, and Tops.

Canal

Moderate lumbar stenosis, beyond the mild stenosis seen with ligamentous buckling or involution, requires surgical decompression. The accompanying mild instability may be treated with interlaminar devices such as Coflex and Vertiflex.

Moderately severe stenosis with more accompanying instability may be treated with the stiffer pedicle-based devices such as Dynesys, PEEK rods, or Isobar.

Osteopenia

Bone quality is the overarching consideration for any device implantation. Osteopenia and osteoporosis are typically contraindications to most device usage. Dynamic stabilization technologies in general have reduced forces at the bone–screw interface when compared to traditional fixation techniques. This may result in a more suitable construct for stabilization in such instances of poor bone quality.

Conclusion

The aging spine is less adaptive to, and less forgiving of, less than ideally applied solutions to pathological conditions. Hence there is an even greater need for thoughtfulness of approach, broad spectrum of choice, and proper selection of technique.

Posterior dynamic stabilization of the lumbar spine offers promising solutions to a variety of challenging clinical scenarios. In most instances, inadequate clinical data exist to fully evaluate appropriateness and efficacy in specific pathological entities. Rigid clinical science is needed to support what is presently clinical intuition. Only through patient trials and physician experience will the proper use of and indication for these devices become clear. Also, further evolution of proper diagnostic techniques that specifically evaluate the dynamic elements of spine pathology will be needed to guide proper selection of technology.

References

1. Anderson, et al. J. Neurosurg.. 2006 Jun;4(6):463-471.

2. Park S.C., et al. J. Korean. Neurosurg. Soc.. 2009 Oct;46(4):292-299.

3. Stoll T.M., et al. Eur. Spine J., 11 Suppl 2:2002 Oct, S170-S178.

4. Neurosurg. Rev., 32, 3:2009 Jul, 335-341, discussion 341–2

5. Zucherman, et al. Eur. Spine J.. 2004 Feb;13(1):22-31.

6. Spine, 30, 12:2005 Jun 15, 1351-1358, 2004

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